1. Field of the Invention
This invention relates to in-line color cathode ray tubes, and more particularly to conductive means disposed on discrete external areas of the tube's glass envelope to dissipate the deleterious charge build-up occurring in the neck portion during tube operation.
2. Background Art
Color cathode ray tubes (CCRTs) conventionally utilized in color television and related pictorial and data display applications employ a screen of red, blue and green cathodoluminescent phosphor elements disposed on the interior surface of the viewing panel portion of an encompassing glass envelope. The envelope comprises an integration of the face panel portion with funnel and neck portions. An electron gun assembly, positioned in the neck portion, directs three in-line electron beams toward the screen, one for each phosphor color. A multi-opening mask member or aperture mask, positioned within the panel and spaced from the screen, permits the electron beams to strike the appropriate elements on the screen, whereupon the elements are excited to luminesce in one of the primary red, blue or green colors.
The gun assembly, encompassed within the neck portion, is constructed of three in-line cathodes to emit three in-line electron beams and a plurality of spatially-related electrodes positioned forward of the cathodes, each having three in-line apertures, to form, focus and accelerate the electron beams. In such in-line gun assemblies, the electrode members are necessarily markedly wider in the direction of the in-line plane than normal to the in-line plane, in order to accommodate the three in-line apertures. As a result, the two side apertures, as well as the surrounding electrode material defining such apertures, are much closer to the interior wall of the tube neck than the central aperture and its surrounding electrode material.
Such in-line gun assemblies have almost totally replaced the older delta gun arrangements in which three separate guns were arranged in a delta configuration.
On the exterior surface of the tube envelope, an electrically conductive coating is disposed on the forward area of the funnel portion. This coating is normally at ground potential when the tube is in operation. Spaced rearward from this funnel coating is a deflection yoke oriented to surround the envelope at substantially the transition area between the funnel and neck portions. Magnetic fields produced by the yoke provide for deflection of the electron beams to effect scanning of the screen raster.
While the glass of the neck portion is normally considered to be electrically neutral, when the tube is put into operation, it has been found that a deleterious charge build-up can occur on the external surface of the neck wall, which can be as high as about 16 KV in magnitude, or higher. Such a charge can induce fields which have a detrimental influence on the trajectories of the side aperture-related electron beams. This influence is evidenced, for example, by a spreading or mis-convergence of the side-related beams resulting in a haze-like appearance of the display image in the peripheral area of the screen and general mis-convergence. When such spreading occurs gradually, over time, it is known as convergence drift.
Although neck charging has been known to occur for some time, recent advances in CRT technology have caused its effects to increase in both magnitude and importance. The desire for improved screen images has led to more stringent requirements for beam convergence, which is related to both resolution and color purity. For example, in certain high resolution tubes, mis-convergence errors as little as one-fourth the width of a phosphor element are considered undesirable. However, the trend toward smaller (mini) necks and yokes to save both materials and deflection power has led to smaller gun assemblies and diminished clearances, magnifying the effects of neck charging.
In addition, the introduction of the internal magnetic ring (IMR) into the mini-neck tube may also enhance neck charging. The IMR is located in the top or convergence cup of the electron gun and sets up magnetic fields which, during tube operation, correct certain mis-convergence problems which would otherwise occur as a result of the tube-to-tube variations inherent in any mass production process. Each IMR is "custom-magnetized" to correct its tube's particular mis-convergence, using an apparatus which closely surrounds the tube neck. Although the apparatus is thought to act as a ground when in place, subsequent rapid neck charging has been observed to occur when the apparatus is removed.
Attempts to reduce the neck charge by the use of grounded internal conductive neck coatings, although in some degree successful, have in general not been satisfactory due to the close proximity of various high potential elements such as gun electrodes and the internal conductive funnel coating, which promotes undesired arcing.
In U.S. Pat. No. 3,746,904 an external circumferential band of grounded conductive coating was applied to the neck of a delta tube over the focusing lenses of the electron gun assembly to evenly distribute the neck charge and prevent convergence drift.
However, it has been found that the known prior art techniques do not adequately resolve the annoying convergence drift problem resulting from neck charge build-up in state-of-the-art tubes incorporating plural beam in-line gun assemblies, particularly tubes having small neck configurations (e.g., mini-neck), and in tubes having internal magnetic rings (IMR).